1. Field of the Invention
The present invention relates to a control circuit of an LLC resonant converter and an LLC resonant converter using the same, and more particularly, to a control circuit of an LLC resonant converter capable of obtaining a desired input/output DC voltage gain with corresponding to a variable AC input voltage and varying a minimum switching frequency without an active power factor correction and an LLC resonant converter using the same.
2. Description of the Related Art
Recently, according to the development of multimedia technology, there have been drastically developed in the technology field for the image and audio using a home theater system.
Particularly, in the audio part, there are many efforts to make sounds to be more realistic. From the computer sound which plays only simple rhythm, there are needs of functions capable of playing the sound of person, various sound effects and the sound being near at the original sound and the sound capable of producing more spectacular and realistic atmosphere.
The sound started from a mono-sound has been now developed to a 3D sound technology capable of a feeling similar to a real world deviated from a simple 2D sound. And, as being developed from a speaker environment of 2 channels to a virtual surround technology capable of reproducing the sound of 4 channels or 5.1 channels, there has been a trend that the interests for a dual power supply and power consumption are increased.
According to such trend, it is needed to a technology development for the miniaturization and slim of the power supply system, a high efficiency/high performance and an improvement of price competitiveness.
FIG. 1 is a diagram showing a conventional fly-back converter, and FIG. 2 is a diagram showing main operational waveforms of the conventional fly-back converter shown in FIG. 1. As the fly-back converter is an insulating type of a Buck-Boost converter, main operations and a converting ratio between input and output voltages are equal to those of the Buck-Boost converter except a transformer turn ratio, if the transformer turn ratio is defined N=(NP/NS), an input and output relationship becomes as follows:
  M  =                    V        O                    V        in              =                  D                  (                      1            -            D                    )                    ⁢      n      
A structure of the fly-back converter is very simple as shown in FIG. 1, since a transformer reset is automatically implemented by the output voltage; it is adapt to construct medium-sized low-cost power supply without requiring an additional reset circuit.
However, as shown in FIG. 2, the voltage shift stresses of the switch and output diode are very large as Vin+NVO and Vin/N+VO respectively, as well as, due to the energy stored at the transformer leakage inductor Llkg during turn-off, a critical voltage ringing is generated at both ends of the switch, in this result, there is a problem to require an additional Snubber circuit for absorbing this.
Meanwhile, since the magnetizing current of the transformer has an offset of a load current size, the transformer utilization is low, when the operational frequency is high and the load capacitance is large, an additional auxiliary circuit must be added for a soft switching, there is a shortcoming that the output voltage has a large riffle since the output current is discontinuous.
Specifically, as described the above, since the output diode has the very large voltage stress as Vin+NVO as well as by the resonance between the transformer leakage inductor Llkg and the output diode junction capacitor the large voltage ringing is added to this, the expensive diode having high withstanding voltage must be used and the Snubber circuit or the like must be included. Accordingly, there are problems that the manufacturing cost of the system is increased and the efficiency is deteriorated, in case when a high output voltage is required, such problems are more deepened.